Every test (and there is quite a few) out there shows that a faster clocked dual core is better than a slower quad for gaming.
This is to be expected since the idea of multi-core is still quite new. There are only a few games today that make use of the new ability.
However: A year from now? Multi-core will be the norm and popular old games as well as the new will be programmed to make use of it.
I think it will take more than a year for true multi-threaded games to be common. Today not even one truely multi-threaded game is in the market, and only one game in beta (Alan Wake) has been shown to be multi-threaded. There is a whole lot of issues which have to be considered when making a game truely multi-threaded (load balanced n-threads), so it is far from a trivial task to do. Look at least 2 years before you can find a hand full of topgames using mulitiple cores in any meaningful way, and 5 years before it will be common.
I would say that if you are going to buy a gaming PC this year, then get a fast dual core, but if you are more a general power user then get a quad.